Magnetite particles prepared through an aqueous solution reaction have recently found wide use as a material of magnetic carriers or magnetic toners of electronic copiers, printers, and the like. While various general development characteristics are demanded in these applications, higher characteristics have come to be required with the latest rapid advancement in copiers and printers using digital technology as boosted by the development of electrophotographic techniques.
That is, the copiers and printers are now required to output high quality images such as graphics and photographs as well as letters. Copiers and printers with a resolution of 1200 dpi or higher have come on the market, and the latent image formed on a photoreceptor is becoming much denser. Hence it has been keenly demanded to develop an imaging technique excellent in fine line reproduction and imaging stability.
In order for magnetic carriers and magnetic toners to meet the requirements, to have high electrical resistance is among the important characteristics required of iron oxide particles. The Applicant of the present invention discloses in JP-A-2000-239021 a technique pertaining to such iron oxide particles, in which the surface of iron oxide particles is coated with an Al/Fe complex oxide layer. According to the disclosure, highly resistant iron oxide particles are obtained. Although the technique succeeds in achieving high resistance, it has a disadvantage that the high resistance cannot be reached without thickening the Al-containing complex iron oxide layer, which turned out to result in reduction of saturation magnetization.
On the other hand, to have a high saturation magnetization and minimized susceptibility to reduction in saturation magnetization is also an important feature required of iron oxide particles for obtaining stable images in electrostatic copying applications. A magnetic carrier comprising iron oxide particles with high saturation magnetization is capable of forming a stable magnetic brush on a magnetic roller in a two-component development system. A magnetic toner comprising iron oxide particles with high saturation magnetization is expected to contribute to stable toner layer formation on a magnet roll thereby suppressing fogging on development. However, the state-of-the-art iron oxide particles are unsatisfactory in resistance against deterioration of saturation magnetization with time and by the environment. While environmental resistance is taken into account in the above-cited technique, since the complex iron oxide layer on the particle surface contains only Al in addition to iron oxide, the iron oxide particles are insufficient in achieving high saturation magnetization and controlling deterioration of saturation magnetization.
Accordingly, an object of the present invention is to provide iron oxide particles which exhibit high electrical resistance and high saturation magnetization, undergo reduced deterioration in saturation magnetization and are particularly suitable for applications, such as magnetic carriers for electrostatic copying and magnetic toners for electrostatic copying, and to provide a process of producing the same.